In general, alumina type abrasive material available until the recent past has been conventionally produced by high temperature smelting of bauxite or Bayer alumina. In such a process the abrasive material is manufactured in bulk form, in large pigs or concentrates. These abrasive pigs are first crushed on what is termed in the art, primary crushers, after which powerful jaw crushers then reduce the lumps to about 3/4 inch size. Roll crushers then break up these small pieces into a usable range from the coarsest to the very finest particle sizes suitable for use in the manufacture of coated abrasive products. The crushed abrasive particles are then separated or screened into individual grades to conform to either CAMI (ANSI/ASC B74.18-1984) or FEPA grading standards. This is accomplished, in general, by passing them over a series of silk bolting cloth screens. These screens are carefully made from threads of exact size and number per square inch to insure extreme accuracy in the size of the apertures through which the abrasive particles are sifted. Mesh size of the screens can vary from 12 mesh (extra coarse) to 600 mesh (extra fine). Subsieve grades, called flours, are graded by hydraulic separators, air classifiers and levitating tanks. The sieve-size grades from the grading screens are regularly checked against standard industry grains. These grades are controlled to match the standard grains by testing on a series of standardized laboratory sieves; whereas flours are checked by a sedimentation method.
Both sieve grades and flours abrasive grains are given numerous treatments and rollings to remove surface contamination and dirt, to increase hardness, and to improve coating and adhesive characteristics. Accurate grading is, of course, all important to eliminate the possibility of damage that could be caused by oversized grains in a coated abrasive product and to provide maximum efficiency of abrasion.
The standards set for abrasive grains for use in the manufacture of coated abrasive products define grain size distribution, e.g. by indicating that the grain particles are larger than a certain size grade or grit, e.g. grade 50, or will pass through that size grade. Actually, the screens used in sizing coated abrasive grains only measure in two dimensions. Thus, abrasive grains that pass through a certain screen, e.g., grade 50, pass such because two of the dimensions of the three dimensional-shaped grain allow for such. Nevertheless an abrasive grain, because it is three dimensional, may be relatively short and stubby or long and skinny even though it passes through a certain grade. Thus, current manufacturing methods for coated abrasive material offer little control of the configuration of the individual abrasive grains used, other than that each has a strong or weak shape. Neither do they offer much control in the lengths of the grains for any particular grade, which can vary greatly. As a matter of fact, any particular grade of abrasive grains used in the manufacture of coated abrasives comprises a certain predetermined amount of overgrade, i.e., larger size, grains and undergrade, i.e., undersized or fines, grains. Thus, roll crushed grains conventionally used in the manufacture of coated abrasive material inherently provides a rather varying three dimensional configuration and a range of sizes for any particular grade. Moreover, because of the roll crushing to grade, there are fines that are necessarily produced that can not be used at all in the manufacture of coated abrasive products. As a result, where such fine abrasive material can not be reworked in the fusing process, such becomes waste, contributing to the overall cost of manufacture of the coated abrasive. Even where such fines are reworked in the fusion process, the fact that such fines are produced in the first place makes such manner of obtaining abrasive grains for use in the manufacture of coated abrasive products less efficient and economical.
Abrasive grains of the alumina type, as above-described, use in the manufacture of coated abrasive products suffer additional disadvantages. They also lack optimum toughness, hardness, and micro structure to achieve efficient grinding in many applications.
A problem which has heretofore been accepted as one of the inevitable characteristics of coated abrasive products is their short lives. Typical useful life times for coated abrasives are measured in minutes, and substantial time can be lost in removing a used-up disk or belt, for example, and replacing it with a new one. Anything which can substantially lengthen the useful life or a coated abrasive fulfills an important, long felt need. Another limitation experienced with known coated abrasive products is that the cutting rate of the product and the surface finish imparted to the work piece varies over the life of the coated abrasive. In general, a new coated abrasive product cuts relatively quickly but gives a relatively rough surface finish to the work piece. Over the life of the product, the cutting rate tends to drop gradually while the surface finish imparted to the workpiece tends to become finer. This makes it very difficult to obtain consistent results in coated abrasive grinding and finishing operations and necessitates the use of operators with more skill and know how than are easily available in many situations.
The heat generated in grinding with a coated abrasive can cause severe "burning" and other damage to the work piece. The heat build-up can be limited to acceptable levels by a combination of grinding at lower pressures against the work piece and by using a freer cutting coated abrasive that requires less applied power. In general, free cutting coated abrasives requiring less applied power and pressure for achieving a given cutting rate have important advantages.
The ability to grind well under low pressure conditions is also very important in applications such as hand held grinding applications such as in the grinding of body welds on automobiles where it is physically impossible or impractical to apply high pressure to the workpiece. In general, the term "low pressure grinding" is used herein to refer to hand held grinding or to grinding at pressures on the order of about 5 lbs./in.sup.2 or less
Sol gel, and particularly seeded sol gel aluminous abrasives have demonstrated substantial advantages over other premium abrasives in broad areas of coated abrasive applications since their introduction some few years ago. Such abrasives are generally made by drying and sintering a hydrated alumina gel which may also contain varying amounts of additives such as MgO or ZrO.sub.2. The dried material is crushed either before or after sintering to obtain irregular blocky shaped polycrystalline abrasive grits in a desired size range. The grits may be later incorporated in abrasive products such as coated abrasive disks or belts.
U.S. Pat. No. 4,314,827 to Leitheiser et al. discloses abrasive grits made by such a method in which the sintered grits contain irregular "snowflake" shaped alpha Al.sub.2 O.sub.3 crystals which are on the order of 5 to 10 microns in diameter. The spaces between the arms of a "snowflake" and between adjacent "snowflakes" are occupied by other phases such as a finely crystalline alumina magnesia spinel.
U.S. Pat. No. 4,623,364, which issued on Nov. 18, 1986 assigned to Norton Company, the assignee of this application, discloses a sol gel method for the manufacture of aluminous abrasive grits, and products other than abrasive grits such as coatings, thin films, fibers, rods or small shaped parts, having enhanced properties. In that patent the conversion of the hydrated alumina to alpha alumina is facilitated by the introduction of seed material into the gel or the gel precursor prior to drying. This can be accomplished by either wet vibrator milling of the gel or gel precursor with alpha alumina media, or by the direct addition of very fine seed particles in powder or other form. To make abrasive grits the seeded gel is dried, crushed and fired. The abrasive grits so produced may be used in the manufacture of products such as coated abrasive disks and grinding wheels. Alternatively, to make shaped parts or rods, the material may be formed or molded as by extrusion before firing. In the case of extrusion, the rods formed are later cut or broken into appropriate lengths.
U.S. Pat. No. 4,744,802, which issued May 17, 1988, also discloses a seeded sol gel process for producing alpha alumina based ceramics useful as abrasive grain and ceramic shaped bodies Such alpha alumina is obtained from alpha alumina monohydrate to which has been added a nucleating agent. Once the gel has formed, it may be shaped, according to the patentee, by any convenient method such as pressing molding or extrusion and then carefully dried to produce an uncracked body of the desired shape. If abrasive material is desired, the gel can be extruded or simply spread out to any convenient shape and dried. After drying, the solid body or material can be cut or machined to form a desired shape or crushed or broken by suitable means, such as a hammer or ball mill, to form abrasive particles or grains.